1. Field of the Invention
This invention relates to a photomicrographic apparatus that takes a picture of an enlarged image of a specimen observed under a microscope.
2. Description of the Related Art
In a photomicrographic apparatus, a photographic apparatus (a camera) is mounted to an observation optical system that optically magnifies a specimen. The photographic apparatus takes a picture of an enlarged image of the specimen magnified by the observation optical system. In some cases, many specimens are observed at a time. In other cases, the observation of the same specimen is made while changing its position. In taking a photograph of such specimens, luminance varies greatly from specimen to specimen observed, as compared with photography by an ordinary photographic apparatus. Similarly, in changing the position of the same specimen, luminance changes. Therefore, taking pictures of specimens requires a photographic apparatus that can measure a very wide illuminance range.
There has been a photographic apparatus suitable for highly luminous specimens. The photographic apparatus is disclosed in Published Unexamined Japanese Patent Applications No. 51-95839 and 2-42426. In those techniques, depending on whether or not the light-receiving surface illuminance of a light-receiving element that senses the luminance at the specimen has reached a predetermined illuminance for insertion and removal of a filter, a light intensity-reducing filter (an ND filter) is inserted in or removed from the photometric optical path to the light-receiving element.
In those techniques, when the light-receiving surface illuminance of the light-receiving element has exceeded the illuminance for the insertion and removal of the filter, the ND filter is inserted in the optical path to the light-receiving element to reduce the light-receiving surface illuminance of the light-receiving element to a photometry enable illuminance range.
With the above techniques, however, the light-receiving surface illuminance of the light-receiving element is close to the illuminance level at which the insertion and removal of the filter takes place. Further, with the techniques, when the specimen is moved for observation, the light-receiving surface illuminance of the light-receiving element increases and decreases with the movement of the specimen.
This means that the ND filter is frequently inserted and removed as the light-receiving surface illuminance of the light-receiving element increases and decreases. As a result, the insert/remove driving mechanism for the ND filter will deteriorate and eventually break down.